Computerized tomography ("CT") scanning systems were initially developed to provide a non-invasive means for viewing internal organs and tissues of the human body. CT scanners have also been developed for industrial applications to allow for non-destructive testing. For example, a CT scanner may be used for viewing cross-sections of structurally critical parts, such as rocket motors or barrels of nuclear or toxic waste. A small imperfection or void in a rocket motor casing can lead to fracturing or cracking during the firing process due to the resultant high stresses imposed on the rocket casing. Voids or unbond defects formed between the liner of a rocket motor and solid propellant contained therein can also create problems when the rocket is fired.
For example, when a rocket motor contains voids or unbond defects, it can fire in an unintended manner. This can lead to error in the velocity of the payload and, more importantly, to error in the direction of travel of the payload. Although guidance systems, using gyroscopes, can correct for small errors, those systems are often incapable of correcting large errors. As a result, the payload can arrive at the wrong location at the wrong time.
Nuclear and toxic waste present yet another industrial dilemma as the waste can corrode and weaken storage containers. Therefore, barrels containing such waste need to be analyzed periodically to ensure that the integrity of the barrel wall remains intact. Failure can create extremely hazardous situations.
As small flaws in any of these applications can lead to disastrous results, an inspection system needs to be both sensitive and accurate. However, pursuant to normal economic constraints and the higher rates of production associated with industrial applications, industrial scanners need to be relatively fast and yet inexpensive.
Typical CT scanning involves directing a source of radiation at a detector while simultaneously interposing the object to be scanned between the radiation source and the detector. The radiation source emits a beam of radiation that penetrates and passes through the object. Depending on the density, size and composition of the object, the beam is attenuated to varying degrees as it passes through the object. The resultant levels of radiation intensity are received by the detector and recorded by a computer. This basic concept has been modified and adapted over a period of time resulting generally in four generations of medical and industrial CT scanners.
The first generation of CT scanner uses only one x-ray detector. A pencil x-ray beam is formed between an x-ray source and the single detector. The pencil beam is traversed through the object or patient to be scanned. The detector receives and measures the variant intensity of the pencil beam and transmits the measured values to a computer that collects and analyzes the data. The pencil beam is then translated a small increment along the length of the object and makes another parallel pass through the cross-section of the object.
For complete image reconstruction of the object being scanned, a multiplicity of parallel passes must be made by the beam. After the desired number of parallel cross-sectional passes are made, the pencil beam is rotated by a small angle, typically one degree, and the entire translational measurement process is repeated. Complete image reconstruction, therefore, involves repeating the translational process until a complete 180 degree scan is completed. For example, the translational process is repeated 180 times for a one degree angle of rotation. With this arrangement, a lot of time is spent in starting and stopping the machine between each traverse and each rotation. With a first generation scanner, therefore, the time for scanning a object is extremely long and is prohibitive for almost all industrial applications.
Although scanning time severely limits the applications of the first generation scanner, it has some advantages. Because only one detector is utilized, the reconstructed image produced by this system does not have any aberrations or artifacts. In systems that use more than one detector, data is collected by different detectors which may receive and report slightly different values for the same intensity of radiation. When the data is reconstructed into an interior image by combining the data from different detectors, artifacts may appear which reflect nothing more than detector variance. Thus, cross-sectional images reconstructed from a scanner using only one detector are devoid of all artifacts and the resulting reconstructed image is of high quality.
A second generation scanner uses a fan shaped x-ray beam of about 30-60 degrees and a plurality of detectors arranged along a line. The plane formed by the beam lies parallel to the cross-section being analyzed. The fan beam is passed through the object and is received by the detector array. In scanning an object, the leading edge of the fan beam, defined by the signal received by the first detector, must begin its scan at the outside edge of the object and the trailing edge of the fan-beam, or signal received by the last detector, must exit the object. Therefore, extraneous data is collected at the beginning and end of each pass as the majority of detectors receive signals completely outside the object. Collection of extraneous data consumes data space and computer time and needlessly increases scanning times.
When activated, a second generation fan-beam makes one cut across the object. The object is then rotated relative to the beam by the angle of the fan-beam and another cut is made and the data recorded. For example, a typical 30 degree fan beam requires 5 rotations to complete a 180 degree scan. When compared to a first generation scanner, the overall scan time is greatly reduced. However, as with first generation scanners, a great deal of time is consumed in starting and stopping the scanner between each traverse, and in collecting extraneous data.
Although quicker, second generation scanners can produce some artifacts in the reconstructed images. Since multiple detectors are used, there are variations between the responses recorded by the detectors. These variations produce small mismatches at different angles of the entire data set. When reconstructed, therefore, the image can contain artifacts. Although computer software can remedy this problem by making corrections for detector variations, the artifacts are never completely eliminated.
Third generation CT scanner systems use an x-ray source which directs a fan-shaped beam of radiation at a planar array of detectors usually, but not always, arranged in an arc. The cross-section of the object to be scanned fits within the fan-shaped beam so that the entire cross-section is covered by the beam. The x-ray source and detector array are then rotated relative to the object until a data set is collected. For a subsequent CT slice, the object is traversed along its length and another set of data is generated. This type of scanner eliminates translation of the x-ray beam or object during the data collection for a single CT slice. In essence, the x-ray source and detector array only rotate during the collection of data for one CT slice. The total scanning time is therefore greatly reduced as no traverse is required.
However, third generation systems also have some limitations. For instance, the size of the object to be scanned is limited by the size of the beam, as the object must fit within it. If large objects are to be scanned, a greater number of detectors is required and the cost can become prohibitive. Conversely, if the object is smaller than the fan beam, extraneous data is collected by detectors receiving radiation signals passing outside the object. Unused data occupies data space and extends computation and scanning times.
In third generation systems, the detector variations can create very significant artifacts in the reconstructed images. Many times, third generation systems are not used in industrial applications due to the pronounced circular artifacts. In addition, the total number of rays collected is limited by the number of detectors in the detector array. In some cases, this can limit the spatial resolution of the system. To obtain better image resolution of the reconstructed image, one must add more detectors to the array. This procedure can be costly.
A fourth generation system consists of an array of detectors positioned in a circle. A large number of detectors is needed to obtain reasonably good resolution and the system can be very expensive. The object is positioned within the circle and a radiation source emits radiation that penetrates the object and is received by the detector array. Only objects fitting within the x-ray fan beam can be scanned with this type of system. Conversely, if a small object is scanned, the detectors collect extraneous data from the radiation beams falling outside the object. The total number of views collected during the data collection is limited by the number of detectors in the detector array. In some cases, this can limit the spatial resolution of the system. To obtain better image resolution of the reconstructed image, one must add more detectors to the array. This procedure can be very costly.
As disclosed in U.S. Pat. No. 4,989,225, CT scanners have also been developed which simultaneously rotate and translate an object during a scan. This process makes it unnecessary to perform sequential passes or to relocate the source and detectors between passes to complete a scab. However, although throughput is greatly increased, multiple detectors are still utilized to image a cross-section and the reconstructed image contains artifacts. Furthermore, this process must be repeated for different cross-sectional slices because the data is collected for one slice at a time. To collect the data for the entire volume of the object, one must collect data for many individual slices.
In conclusion, all of the previous generations of CT scanners have one or more of the following disadvantages: prohibitive scanning time, object cross-section size limitations, artifacts in cross-sectional images, extraneous data collection, or detector dependent image resolution.
It is an object of this invention, therefore, to provide a CT scanner capable of single-pass, full-volume scanning. It is also an object of this invention to provide a CT scanner capable of accommodating various cross-sectional size objects without corresponding detector array variations or extraneous data collection. Yet another object of this invention is to provide variable image resolution not dependent on the detector array configuration. Finally, it is an object of this invention to eliminate or reduce reconstructed cross-sectional image artifacts.